Low temperature surface treatment of ferrous metals with nitrogen is well known in the art and is generally referred to as nitriding. In addition, the low temperature surface treatment of ferrous metals with nitrogen and carbon is also well known and is generally referred to as nitrocarburizing.
When a ferrous metal is exposed to raw ammonia at temperatures below 1300.degree. F., the surface of the metal will absorb nascent nitrogen created from the dissociation of the raw ammonia at the metal surface. In addition, if a hydrocarbon gas is also present, the metal surface will absorb a small amount of carbon from the breakdown of the hydrocarbon. The amount of nitrogen that the metal surface will absorb is a function of the amount of raw ammonia which reacts directly at the surface. The nitrogen which the surface absorbs combines with the iron (and other alloying elements, if present) in the metal and forms iron-nitrogen compounds (and alloy-nitrogen compounds, if present). As more nitrogen is absorbed, the nitrogen will diffuse inward into the metal, creating a gradient of nitrogen content in the surface of the metal. The concentration of nitrogen throughout this gradient will dictate the structure of the iron-nitrogen compounds that are formed.
Conventional low temperature surface hardening is typically carried out at temperatures below 1090.degree. F. in either molten salt baths or recirculating gas-type furnaces. The temperature of these processes is limited to below 1090.degree. F. in molten salt baths due to the melting points and chemical stability of the salts. In addition, salt baths are limited to a particular chemistry and therefore, will only produce one type of nitrogen gradient in the metal surfaces exposed to the salt.
Gas-type equipment is limited to temperatures below 1090.degree. F. due to the catalytic effect of the furnace heater tubes or retort walls on the dissociation of the raw ammonia. In order for the ammonia to produce the desired effect on the metal, it must break down at the metal surface. If it breaks down anywhere else in the furnace, it will not affect the metal to be treated. In conventional furnaces, either heater tubes or retort walls (depending on the type of furnace) run at a higher temperature than the furnace work zone. The ammonia will preferentially break down at the higher temperature areas in the furnace. If the furnace is below 1090.degree. F., the typical atmosphere flow through rate of the furnace supplies sufficient ammonia to break down both on the metal to be treated and at the heater tubes or retort walls; however, at higher temperatures, since the atmosphere flow through rate remains constant, yet there is more ammonia breaking down in other areas of the furnace, the metal becomes "starved" for ammonia. The flow through rate in a conventional furnace cannot be readily varied because of the design of the furnace. The furnace is designed for a specific flow rate plus or minus, e.g., 10%. If one goes below the design flow rate, too much oxygen will enter from the exterior and there is a danger that the furnace may explode. If one goes above the design flow rate, the doors, vents and seals may blow off. Since these processes are limited in temperature and flow rate, and due to the requirements for high volumes of raw ammonia (to insure that the metal is not "starved" for nitrogen), there is very little flexibility in atmosphere and structure composition of the treated metal. In addition, it is known that any flammable atmosphere below auto ignition temperatures presents a safety hazard in this type of equipment due to the natural leakage and therefore oxygen infiltration.
Fluidized bed furnaces are also known. A fluidized bed furnace basically comprises a retort filled with an inert, sand-like material (typically aluminum oxide) and a method for heating the retort. A gas is passed through a diffusion plate at the bottom of the retort which acts to evenly distribute the gas flow over the bottom of the bed of aluminum oxide. At a certain gas flow rate, the velocity of the gas stream exceeds the drag force exerted by the particles of aluminum oxide and causes them to float (or be suspended) in the gas stream. At this point, the particles are fluidized and will take on many properties of a liquid.